Differential scanning calorimetric studies (Donovan and Mihalyi, PNAS 71, 4125, 1974) suggested a slow action of thrombin upon fibrinogen, distinct of the liberation of fibrinopeptides. Attempts will be made to elucidate the nature of this action. Possibly, it is directed toward the removal of the tripeptide Gly-Pro-Arg from the Ax-chain (Iwanaga et al. Eur. J. Biochem. 8, 189, 1969).